Anti-roll mechanism for a rack and pinion

ABSTRACT

A rack and pinion apparatus comprising a toothed rack, the tops of the teeth of which lie in a common plane, the rack and pinion apparatus further comprising an anti-roll mechanism having a rack teeth engaging pad mounted to permit relative movement between the rack and teeth engaging means and so as to be in contact, at any given instant, with the tops of at least two rack teeth.

This invention relates to anti-roll mechanisms for rack and pinions andhas been conceived in connection with a rack and pinion for a vehiclesteering mechanism and will primarily be discussed in relation thereto.However, it is to be noted that the invention is applicable to rack andpinions used in other than vehicle steering mechanisms.

A rack and pinion is a widely accepted mechanism for steering motorvehicles and it is recognised that to achieve positive steering, meshclearance (backlash) between the teeth of the rack and pinion needs tobe kept to a minimum. Ideally, zero backlash is required butmanufacturing tolerances, however tight, make this virtually impossiblewithout involving extra components and/or machining operations whichintroduce unacceptable higher manufacturing costs.

It is common to employ a pinion with helical teeth and this gives riseto a force which imparts a rocking motion to the rack. FIG. 1 of theaccompany drawings illustrates a known rack and pinion for a steeringmechanism in which the rack 1 is supported for axial sliding movement ina support 2, the rack being formed from cylindrical stock and thesupport being provided with a corresponding part-cylindrical supportsurface 3. The teeth 4 of the helical pinion 5 mesh with the teeth 6 ofthe rack 1 and on rotation, the pinion generates an axial force F_(AX)along its axis of rotation, which force acts at the pitch line P of therack 1, the distance between the pitch line and the longitudinal axis ofthe rack 1 representing a moment arm M. The rotational force or torque Tacting on the rack 1 is thus:

    T=F.sub.AX ×M

This torque is resisted locally at the ends of the rack teeth 6, givingrise to wear and/or damage. Clearly, this is undesirable especially whenlong term use is essential as in a vehicle steering mechanism.

Attempts have been made to minimise the rocking motion of the rack 1 dueto the torque T, one arrangement being to provide inclined faces on therack to give a general v-shaped cross-section to the rack as illustratedin FIG. 2 of the accompanying drawings. The support 2 for the rack 1 hasa corresponding V-shaped mating surface 3 formed therein. In view of theaccuracy required, the inclined faces have to be machined and it will beappreciated that it is not a simple matter to effect this machining,relative to the rack teeth 6, without tolerance or error and to producelikewise the mating surface 3 on the support 2. Furthermore, additionalerrors are likely to be introduced when assembling the anti-rollmechanism in the housing of the steering mechanism. Consequently, notonly is this anti-roll mechanism relatively expensive to manufacture,but the cumulative errors or tolerances contribute to a reduction inefficiency of operation of the rack and pinion, particularly the freedomof movement of the rack, which is unacceptable.

The primary object of the present invention is to provide an anti-rollmechanism for a rack and pinion which is relatively inexpensive tomanufacture but which is effective without detracting from theefficiency of operation of the rack and pinion.

According to the present invention, there is provided rack and pinionapparatus comprising a toothed rack, the tops of the teeth of which liein a common plane, the rack and pinion apparatus further comprising ananti-roll mechanism having rack-teeth-engaging means mounted to permitrelative movement between the rack and teeth-engaging means and so as tobe in contact, at any given instant, with the tops of at least two teethof the rack.

Inasmuch as it is common practice to provide flat tops to the rack teethand to ensure that these tops are co-planar, then the present inventionmakes use of this existing "surface" to provide the means for preventingrolling or rocking motion of the rack, thus dispensing with therelatively expensive machining or manufacturing equipment discussedabove.

The teeth-engaging means of the anti-roll mechanism may comprise ahardened pad, preferably supported in a carrier which is provided withmeans for adjusting the pad relative to the rack teeth in order toaccommodate manufacturing tolerances. The pad may be part cylindricalwith the carrier having a complementary part-cylindrical bearing surfaceto permit relative movement between the pad and carrier for alignmentpurposes.

Instead of a pad, the teeth-engaging means may comprise one or morehardened rollers, each roller being arranged to be in engagement, at anygiven instant, with the tops of at least two rack teeth, the rack teethnecessarily being skewed with respect to the longitudinal axis of therack for anti-roll rollers to be employed. If the amount of skew of therack teeth is great enough, it is possible to employ a single rollerbecause the latter will not be able to fall into the space betweenadjacent teeth which, if this were to happen, would negate the anti-rollfunction of the roller. When the skew of the rack teeth is notsufficient for a single roller to be employed, then two rollers may beused which are spaced apart such as to ensure that any given instant, atleast one of the rollers is in contact with the flat tops of at leasttwo teeth even if the other roller is located over the space betweenadjacent teeth. The or each roller may be mounted in a carrier which ismounted relative to the rack such that a roller cannot engage the spacebetween two teeth.

The anti-roll mechanism of the present invention may be employed inaddition to, or instead of, the normal means employed to adjust the rackwith respect to the pinion in order to set up the desired meshingrelationship between the two.

Rack and pinion apparatus embodying the present invention will now bedescribed in greater detail, by way of example, with reference to theaccompanying drawings, in which:

FIGS. 1 and 2 are diagrammatic illustrations of prior art rack andpinions,

FIG. 3 is a diagrammatic side view of one embodiment of the presentinvention,

FIG. 4 is a plan view of FIG. 3, and

FIG. 5 is a cross-sectional view of a second embodiment of the presentinvention.

Referring first to FIGS. 3 and 4, the first embodiment of the presentinvention by way of a rack and pinion apparatus comprises a rack 1, arack support 2 and a pinion 5 with helically cut teeth 4, thesecomponents being similar to those described above with reference to FIG.1 of the drawings. It will be seen that the teeth 6 of the rack 1 areprovided with flat tops which are coplanar. The rack 1 is formed fromcylindrical stock and conveniently, the teeth are formed in seriesproduction with a single-pass broach which cuts the root flanks and topsdirectly into the rack bar, whereby the tops of the teeth 6 being inaccurate alignment, i.e. co-planar.

In accordance with the present invention an anti-roll mechanism 20 isprovided comprising an anti-roll pad 7 of a hardened metal, such assteel, the pad having a part-cylindrical cross-section and beingreceived in a pad-housing 8 which has a complementary part-cylindricalbearing surface 9, the pad being received therein in a manner whichpermits relative movement between these two components about thelongitudinal axis of the pad 7 but not along that axis. The housing 8 iscylindrical but is provided with a pair of flats 11 at the end oppositeto that in which the bearing surface 9 is formed and by which thehousing is mounted, non-rotationally, in a key plate 12. Disposed overthe key plate 12 is a closure plate 13 which is bolted, using the holes10, to a housing of the steering mechanism of which the rack and pinionapparatus forms part but which is not shown in FIGS. 1 and 2 of thedrawings. The closure plate 13 has a tapped hole 14 provided thereinwhich receives an adjustment screw 15 by which the engagement of theanti-roll pad 7 with the rack teeth 6 is set and then locked intoposition by a lock nut 16.

When the rack and pinion 1 and 5 are brought into basic meshingrelationship, fine adjustment of this relationship is set by adjustmentmeans associated with the rack support 2, this adjustment means notbeing shown in FIGS. 1 and 2 but being illustrated in FIG. 5 to whichreference will be made hereafter. Following this adjustment, theanti-roll pad 7 is then adjusted into close contact with rack teeth 6,without overcoming the spring force of the adjustment means of thesupport 2, using the adjustment screw 15, the latter then being lockedwith the lock nut 16 in the selected position. In operation, the pinion5 is rotated about a fixed axis and on rotation, moves the rack 1 to theleft or the right, depending on the direction of rotation, this rackmovement effecting steering movement of the vehicle and being augmented,in a power-assisted steering mechanism typically by hydraulic power. Therack 1 slides beneath the anti-roll pad 7, the latter serving torestrain the rack from any rocking movement about its longitudinal axisas a result of the torque imparted by the rotation of the pinion 5 asdescribed above. It will be seen from FIG. 3 that the anti-roll pad 7spans three of the rack teeth 6, and from FIG. 4 that the pad extendssubstantially across the full width of the flat top of the rack teeth 6,bearing in mind that although the roots of the rack teeth extend thefull width of the rack, the tops are curtailed, in conventional manner,because of the use of cylindrical stock for the rack. It will be seenthat the reaction force to the torque T is now between the top of theteeth 6 and the pad 7, rather than the teeth flanks and the pinion 5.Although the pad 7 needs to contact a major part of one tooth 6, it needonly contact a minor part of one or more adjacent teeth.

Turning now to FIG. 5, this shows part of a power-assisted vehiclesteering mechanism comprising a housing 17 which is of cast constructionand in which the rack 1 is mounted for sliding movement, the pinion 5being shown only in outline. The right-hand end of the rack 1 is engagedby a ram 18 fitted with a piston ring 19 slidably mounted in a cylinder21 fitted as a continuation to the housing 17. Hydraulic fluid isadmitted to one side or other of the piston ring 19 in order to powerassist movement of the rack 1, as appropriate.

The support 2 for the rack is provided with a recess 22 at the endopposite to that on which the bearing surface 3 is formed, this recessreceiving a spring 23 over which is fitted a retaining plate 24. Therecess 22 is closed by a closure plate 25 bolted to the steeringmechanism housing 17. The closure plate 25 is provided with a tappedhole 26 which receives an adjustment screw 27 provided with a lock nut28, the adjustment screw being used to adjust the meshing relationshipbetween the rack 1 and pinion 5 as discussed above. On the side of thehousing 17 opposite to the support 2 there is provided the anti-rollmechanism in accordance with the present invention which, as will beseen, is located immediately next to that part of the housingaccommodating the pinion 5. The anti-roll mechanism 20 of thisembodiment, of the invention comprises the same basic components as theanti-roll mechanism of FIGS. 3 and 4 but in this embodiment, theanti-roll pad 7 is provided with an extension 29 opposite the face incontact with the rack teeth 6, this extension being provided with a slot31 and also receiving a collar 32 serving to retain a pin 33 within theslot 31. The pin 33 serves to prevent rotation of the anti-roll pad 7about a vertical axis but allows movement about the longitudinal axis ofthe pad, as in the embodiment of FIGS. 3 and 4, to accommodatemanufacturing tolerances. Thus, in this embodiment, the key plate 12 andclosure plate 13 of the embodiment of FIGS. 3 and 4 are dispensed with.The adjustment screw 15 is threadedly received directly in a collar 34provided in the housing 17 and is locked into position using a lockingcompound, for example. The setting up and operation of the anti-rollmechanism is as described with reference to FIGS. 3 and 4 of thedrawings.

It will be appreciated that the anti-roll mechanism 20 of FIGS. 3 and 4can be used in place of the mechanism 20 of FIG. 5.

The support 2 and the anti-roll pad housing 8 are conveniently formedfrom a PTFE-loaded material, such as a synthetic plastics material, inorder to provide a bearing surface for the rack with a very lowcoefficient friction. Lubrication for the rack with respect both to thesupport 2 and the pad 7 is provided by grease, or other lubricant,provided within the housing 17 and being allowed to circulate around thecomponents contained therein by virtue of the rack 1 being provided witha through bore.

It will be seen that the present invention provides a relatively simplebut highly effective anti-roll mechanism for the rack of a rack andpinion apparatus and thus represents a significant advance in the art.

We claim:
 1. Rack and pinion apparatus comprising a toothed rack havinga plurality of teeth, the teeth having tops which lie in a common plane,an anti-roll mechanism having rack-teeth-engaging means mounted topermit relative movement between the rack and rack-teeth-engaging means,with the rack-teeth-engaging means being in contact with the tops of atleast two of the teeth of the rack to restrain rolling of the rack aboutits longitudinal axis.
 2. Rack and pinion apparatus comprising a toothedrack having a plurality of teeth, the teeth having tops which lie in acommon plane, an anti-roll mechanism having rack-teeth-engaging meansmounted to permit relative movement between the rack and therack-teeth-engaging means with the rack-teeth-engaging means being incontact with the tops of at least two of the teeth of the rack torestrain rolling of the rack about its longitudinal axis, therack-teeth-engaging means comprising a pad supported in a carrierprovided with means for adjusting the pad relative to the teeth of therack.
 3. Apparatus according to claim 2, wherein the pad is mounted inthe carrier for relative movement with respect to the carrier about atleast one axis.
 4. Apparatus according to claim 2 or 3, wherein the padis part cylindrical.
 5. Apparatus according to claim 2, wherein thecarrier is mounted for non-rotational movement about an axis generallyperpendicular to the longitudinal axis of the rack.
 6. Apparatusaccording to claim 5, wherein the carrier is provided with a pair offlats for non-rotationally mounting the carrier in a key plate attachedto a housing for the apparatus.
 7. Apparatus according to claim 5,wherein the pad is provided with a slot or recess on a side of the padopposite to a face of the pad engageable with the rack teeth, the slotor recess receiving an elongated member retained in position by a collarfor non-rotationally mounting the carrier.
 8. Apparatus according toclaim 2, wherein the adjusting means comprises a screw.
 9. Apparatusaccording to claim 1 or 2, further comprising means for relativeadjustment of the rack and pinion.
 10. Rack and pinion apparatuscomprising a toothed rack having a plurality of teeth, the teeth havingtops which lie in a common plane, an anti-roll mechanism havingrack-teeth-engaging means mounted to permit relative movement betweenthe rack and rack-teeth-engaging means with the rack-teeth-engagingmeans being in contact with the tops of at least two of the teeth of therack to restrain rolling of the rack about its longitudinal axis, withthe rack-teeth-engaging means comprising roller means.
 11. Apparatusaccording to claim 10, wherein the roller means comprises a singleroller oriented generally transverse to the rack and in engagement withthe tops of at least two of the teeth of the rack.
 12. Apparatusaccording to claim 10, wherein the roller means comprises a plurality ofrollers spaced apart for maintaining at least one roller in engagementwith the tops of at least two teeth of the rack.
 13. Apparatus accordingto claim 10, further comprising means for relative adjustment of therack and pinion.